Microporous membranes with pore size varying from 0.01 to 10 microns are known in the literature, but are generally easily fouled and/or relatively hydrophobic, or weak and hydrophilic, compacting at high pressures and difficult to produce with high porosity and/or well defined surface pores.
Prior art method of microporous membrane production are numerous. Some examples will be given in the description which follows.
(1) A solution of polymer is cast with subsequent solvent removal under carefully controlled conditions. Such removal must be carried out very slowly, which results in an expensive process of poor reproducibility, with a wide variation of pore size for a given membrane. Carefully controlled conditions of temperature, humidity and air circulation must be maintained.
(2) A well-defined narrow range of pore sizes may be obtained by a nuclear penetration and etching technique. The pore volume is very low however (less than 15%), in order to minimise degradation. In addition, many polymers are unsuitable for use in this process, because of poor etching characteristics.
(3) Ultraviolet and electron beams may be used to polymerize monomers into a three dimensional cross-linked microporous membrane (U.S. Pat. No. 4,466,931). The formation of pores occurs during the polymerization process, whereby the polymer precipitates from the solution of monomer. While this process may achieve rapid production rates, the membranes are isotropic and more easily fouled than asymmetric structures. In addition, filters of 1 to 10.mu. have not been reproducibly obtained, and there is a problem of leachable materials.
(4) A solution of a membrane forming polymer may be quenched in a polymer nonsolvent, e.g. water (Marinaccio et al U.S. Pat. No. 3,876,738 and Pall, EP No. 0 005 536). Many different materials such as cellulose derivatives, polyamides, polysulfones and polycarbonates have been used, and these membranes are generally characterized by asymmetric structures. Control of pore size is generally a function of careful formulation of the polymer solution, which is brought to the point of incipient precipitation and/or the use of a gelatin bath containing a high percentage of polymer solvent. Careful stirring, temperature and rates of quenching are mandatory for a uniform product. It is difficult, however, via this method to achieve uniform microfilters with pores sizes reliably in the range of 1 to 10.mu.. It is also a costly process to carry out, with a large expenditure for solvents.
(5) Sintering of particles of organic materials such as polyolefins (e.g. high density polyethylene), polytetrafluoroethylene, polyvinylidene fluoride, and inorganics such as alumina, silica, zirconia, graphite and other forms of carbon have produced useful microlfilters. In the case of organic polymers, it has been difficult to achieve uniform and narrow pore size distribution and the membranes have been isotropic rather than asymmetric. Inorganic membranes have achieved asymmetry by sintering two or three layers of different particle sizes. These membranes foul relatively easily and are considerably more expensive than organic polymers.
(6) Microporous membranes have also been made by a microcracking process (Ind. Eng., Prod. Res. Develop., Vol. 13, No. 1, 1974). For example, deformation of annealed polypropylene parallel to the direction of extrusion and followed by high temperatures for stabilization of the elongated films, gives rectangular pores (for example 2000 A and 4000 A by 200 and 400 A, respectively). However, in order to maintain mechanical strength, these membranes do not have a high density of pores. In addition, these membranes are isotropic and demonstrate relatively low flux.
(7) Yet another popular technique is the phase separation of a polymer solution or dispersion. In effect, a polymer is brought into solution or dispersed at elevated temperature and then solidified by cooling and removing the liquid or solvent (U.S. Pat. No. 3,812,224). In one approach (Castro, U.S. Pat. No. 4,247,498) any synthetic thermoplastic polymer may be dissolved in a compatible liquid to form a solution. The plastic is then rendered microporous by cooling at a rate fast enough to prevent liquid-liquid phase separation. The cooling rate criteria must be carefully adjusted to form cellular microstructures of spherical shapes and pores or passageways interconnecting adjacent cells. The basic structure is relatively homogeneous and isotropic. Though this method results in highly porous membranes, the isotropic structure is susceptible to fouling and plugging, and compaction at elevated operating pressures.
Materials commonly used for making microfilters are polycarbonates, polyamides (nylon 6, nylon 6,6, nylon 610, nylon 13), polysulfones, cellulose derivatives (for example, cellulose, cellulose discetate, cellulose triacetate, cellulose nitrate), polyacrylonitrile and copolymers, polypropylene, polytetrafluoroethylene, alumina, silica, carbon, polyvinylidene fluoride, high and low density polyethylene, polypropylene, polystyrene, polyvinyl chloride, acrylonitrile-butadiene-styrene terpolymers, styrene-acrylonitrile and styrene-butadiene copolymers, polyvinylacetate, polyvinylidene chloride, ethylenevinylacetate copolymers, ethylene-acrylic copolymers, polymethylacrylates, and oxidation polymers such as polyphonylencoxide.
The present invention provides microporous membranes and a procedure for making them, which overcomes these shortcomings of the current state of art.